The present invention relates to a transfer apparatus for transferring a toner image formed within an electrophotographic apparatus onto the recording paper or the like transfer medium.
The electrophotographic apparatus is basically configured as follows: First, the surface of a photoconductor is uniformly charged, an electrostatic latent image corresponding to the image to be recorded is formed by exposing the surface of this photoconductor, the electrostatic latent image is developed to form a toner image, and the toner image thus formed is transferred to and fixed on the recording paper.
In the aforementioned processes, a well-known transfer means for transferring the toner image attached on the surface of the photoconductor to the recording paper is divided into contact-type transfer means for performing the transfer operation by electrostatic force with a member charged with electrons (the charged member) kept in contact with the recording paper, and transfer means of non-contact type for performing the transfer operation free of contact with the recording paper.
The transfer means of contact type is classified into the belt transfer system and the roller transfer system by the type of the charged member.
The transfer means of non-contact type, on the other hand, includes a corona transfer system in which a corona ion flow is irradiated directly on the back of the recording paper to which it is desired to transfer toner image.
Of all the transfer systems described above, the belt transfer means, as disclosed in U.S. Pat. No. 3,642,362, JP-A-54-58034 and JP-A-63-83765, is so constructed that the recording paper is transported to a photoconductor portion having a toner image formed thereon, and a dielectric belt charged in the polarity opposite to that of the toner charges is pressed against the photoconductor member portion from the back of the transported recording paper (the side of the recording paper far from the photoconductor member) thereby to transfer the toner image attached on the photoconductor member portion to the surface of the recording paper. A corona charger is used as the above-mentioned means for charging the dielectric belt.
Another example of the construction for belt transfer is embodied by the belt transfer means disclosed in JP-A-59-184377. In this transfer means, an endless transfer belt adapted to rotate with a portion thereof kept in contact with the surface of a photoconductor has the surface thereof charged in advance, and the recording paper is pressed (in close contact) against the surface of the photoconductor while being adsorbed electrostatically to the surface of the transfer belt, thereby transferring the toner image.
The belt transfer system in which the recording paper is pressed against the surface of the photoconductor has the advantage of a high transferability to the thick or rough-surface paper as compared with the corona transfer system described above.
In the aforementioned prior-art systems, the corona transfer system is such that in the case where the recording paper is thick or a portion of the recording paper is deformed, there causes a gap between the recording paper and the photoconductor member. As a result, the distance of movement (transfer) of the toner image is lengthened, so that toner particles are undesirably scattered, thereby leading to the shortcoming of making it impossible to produce a clear transfer image (transfer a clear toner image to the recording paper).
According to the belt transfer system, on the other hand, if the belt material or the recording paper becomes humid under an environment of high humidity and the electrical resistance value thereof decreases, the intensity of the electric field of the transfer position (contact spot between the recording paper and the photoconductor member) increases excessively. This causes a charge current to flow through a guide for transporting the recording paper or the belt drive roller, with the result that the quality of the transfer image or the transfer efficiency is deteriorated. Further, if the dielectric belt is irradiated with the corona discharge for a long time, the surface layer of the belt deteriorates in quality and the electrical resistance of the belt is reduced. Thus the electric charges are liable to leak out and thereby, to have an adverse effect on the charge-holding characteristic.
This effect will be explained with reference to the graph shown in FIG. 21. As seen from FIG. 21, in the case of a high ambient humidity or a long printing operation, the transfer efficiency is deteriorated so that image density is reduced.
The transfer efficiency is given as an evaluation function .eta. defined by equation (1) below. ##EQU1## As a measure for solving the problem of a reduced transfer efficiency or image density, the technique for monitoring the surface potential of the belt and setting it to a predetermined value is disclosed in JP-A-60-57364. The surface potential of the belt, however, is a physical quantity dependent on the resistance value of the belt surface, and therefore the belt surface potential monitored is not a physical quantity reflecting the resistance value of the recording paper.
Also, the transfer belt, which is worn by contact with the recording paper or the photoconductor and is deteriorated in electrical characteristics by the ozone generated from the charger, is required to be replaced at short intervals of time. This takes extra labor of the user and makes the operation troublesome.